The human T-cell leukemia viruses (HTLV) represent a family of T-cell retroviruses with three known members. HTLV type I (HTLV-I) has transforming activity in vitro and is etiologically linked to adult T-cell leukemia, which is known to be endemic in several parts of the world. HTLV-II is another retrovirus having transforming capacity in vitro, and has been isolated from a patient with a T-cell variant of hairy cell leukemia. HTLV-III, which has also been called lymphadenopathy-associated virus and is now known as the human immunodeficiency virus (HIV), is lytic for certain kinds of T cells and has been linked to the etiology of acquired immunodeficiency syndrome (AIDS). Unlike the HTLV-I and -II viruses, HTLV-III is not known to have in vitro transforming activity.
The diagnosis of HTLV-I infection is usually based on serum antibody response to HTLV-I peptide antigens. This usually involves an initial screening assay to identify HTLV-I antibodies, based on an enzyme immunoassay (EIA) with HTLV-I virion peptides. The assays presently used for blood screening detect about 0.5 to 0.05% HTLV-I and HTLV-II positives; of these, about 4 out of 5 are false positives. Therefore, positive sera must be further tested in a confirmatory assay, using Western blot or radioimmunoprecipitation assays which detect antibody reaction to specific HTLV-I peptide antigens.
Current blood testing procedures require confirmation tests based on immunoreaction with HTLV-I p24 gag protein and at least one of the envelop proteins gp46, gp21, or gp68. When the test antigens are prepared from virion proteins, only gp46 gives a high rate of antibody reaction with true HTLV-I seropositives. Even then, the reaction with gp46 may be detected only by additional antigen testing with a more sensitive radioimmunoprecipitation assay. The above screening and confirmation testing identifies HTLV-I and HTLV-II positives, but does not distinguish between the two HTLV viruses. Currently unequivocal differentiation between HTLV-I and HTLV-II infection can only be made by isolating the virus, followed by Southern blot analysis or selective viral nucleic acid amplification by polymerase chain reaction (PCR); however, due to the low level of infected lymphocytes in HTLV-infected individuals, not all HTLV seropositive samples will test positive when analyzed by PCR (Williams). In addition, these techniques are not suited to high volume screening of sera in the clinical setting.
It would therefore be desirable to provide an improved method for detecting and differentiating HTLV-I and HTLV-II positive sera. In particular, the improved test should be capable of detecting all HTLV-I and HTLV-II positive sera, with a minimum number of false positives, and also be able to distinguish HTLV-I from HTLV-II positive sera.